Ozone Continues to Increase in East Asia Despite Decreasing NO2: Causes and Abatements

Abstract

Space-borne ozone (O3) measurements have indicated consistent positive trends across the entire Asia–Pacific region despite the considerable reduction of NOx since 2000s. The rate of increase in O3 derived from lower free tropospheric column measurements was observed to be 0.21 ± 0.05 DU/decade during 2005–2018. Our space-borne-based diagnosis of the nonlinear photochemical formation regimes, NOx-limited and NOx-saturated, show that O3 chemistry is undergoing a transitional process to the NOx-limited regime throughout most of the Asian region. Nevertheless, NOx-saturated conditions persist at present in and over eight major megacities. These NOx-saturated conditions in megacities contribute to the increased O3 due to NOx reduction, which could also affect the enhanced O3 concentrations throughout the Asia–Pacific region via long-range transport. This indicates that VOC limits along with NOx reductions are needed in megacities in the short term to reduce O3 levels. Moreover, NOx saturation in major megacities will continue until 2025, according to the forecast emission scenarios from the Intergovernmental Panel on Climate Change (IPCC). These scenarios gradually shift nearly all cities to the NOx-limited regime by 2050 with the exception of few cities under IPCC RCP8.5. Thus, continued reductions in NOx will be a key factor in reducing O3 in the long term.

Satellite-Based Diagnosis and Numerical Verification of Ozone Formation Regimes over Nine Megacities in East Asia

“…When both NOx and VOCs were reduced (EXP3), BJ, TJ, and BS showed no specific responses, presumably due to the offset caused by almost equal opposite impacts from either NOx or VOC emission reductions. However, HB and SD showed a considerable O3 decrease of −11.5%, whereas SH, SU, and TK unexpectedly showed an increase in O3 of Lee et al (2021) [16] described the spatiotemporal FNRs during 2005-2018 over the same area, including the main megacities, and pointed out the rapid shift from an NOx-saturated to an NO x -limited regime, and the main cause was the lower FNR (FNR = 1.79 ± 0.50) than that of domain average (FNR = 3.66 ± 1.40) due to the rapid decrease in NO 2 since 2013 with steadily increasing HCHO (see Figures 2-4). These tendencies were also consistent with the emission trends of NO x and VOCs [3,4,50].…”

“…Here, FNR is one of the good indicators for differentiating between NOxlimited and VOC-limited regimes; however, it should be noted that the FNR classification may differ depending on the region and time of day [52,53]. We classified FNR < 1.0 as VOC-limited, FNR > 2.0 as NOx-limited, and 1.0-2.0 as transition phases [16,52] in Figure 3. The domain-averaged FNR over China, South Korea, and Japan derived in the current study was 2.46-5.19 during 2014-2018 (Figure 3), and the resultant FNR in the nine meg-…”

“…We analyzed the O 3 partial column (900-700 hPa) data for the period of 2014-2018 that were retrieved from satellite OMI measurements [21]. The retrieval algorithm and other relevant descriptions of the OMI Ozone Profile (OMPROFOZ) research product, NASA OMI standard NO 2 product, and OMI standard HCHO product are described in detail in previous studies [16,[21][22][23]. It should be noted that satellite retrievals of ozone measurements (900-700 hPa) have uncertainties ranging from 6 to 35% in the troposphere [24][25][26][27] and are still in development.…”

“…In addition, the retrieved ozone profiles have also been used to quantify the global tropospheric budget of ozone and evaluate the effectiveness of the current chemistryclimate models in reproducing the observations of Sauvage et al (2007) [33], Zhang et al (2010) [34], and Hu et al (2017) [35]. The detailed OMI tropospheric NO 2 and HCHO column data for the spatiotemporal variations of tropospheric ozone precursors in our analysis were described previously by [16]. In assessing the changes in the OMI measurements and model results, linear regression analysis was employed to identify the mean and standard deviations of the increasing/decreasing rates.…”

Transboundary aerosol transport process and its impact on aerosol-radiation-cloud feedbacks in springtime over Northeast Asia

“…Figure 2 b shows that PM 10 concentrations in the Seoul metropolitan area (SMA) in Korea increased from 1995 to 2009; however, it decreased since the beginning of 2010 (Figure S1 ) owing to the implementation of the Special Air Quality Act in the Seoul metropolitan area by the Korean government in 2003 and 2010, which led to significant declines in PM 10 concentrations over South Korea, including the SMA 36 , 49 . It is interesting to note in Figure S1 that the two starting years of abating policies (2003 and 2010) are consistent with the first and second most significant PM 10 declining trends, respectively (Figure S1 ).…”